The GNU Project has announced a new project called GNU Free Call, an open source Skype alternative that will offer anonymous VoIP and will use the GNU SIP Witch server as the back end. GNU SIP Witch requires a minimum of system resources so it can be used on cell phones too so it seems the goal is to provide a cross-platform application, the immediate target most probably being Android.

Exactly. It is like everybody using MS Office and your OpenOffice exchange is broken.
Everybody uses skype because it works, it passes through routers and works even inside big firewalled company networks.
What would be needed is a skype-compatible implementation.

And, to complete the rant: The latest skype versions are really terrible, they have an indecent UI that looks like iPhone stuff brought to your workstation.